7/27/2019 Axial Flux PM Generator

http://slidepdf.com/reader/full/axial-flux-pm-generator 1/2

Axial Flux Permanent Magnet Generator for Wind Power Applications.

 Asko PARVIAI NEN, Lappeenranta University of Technology, Finland, Pentti KONTKANEN, Kylmätec Ky, Outokumpu, Finland .

Small-scale wind power plantsare an attractive choice togenerate electrical power on

rural areas where the installationof the distribution network is noteconomically reasonable. In suchlocations, e.g. on small islands,wind power plants or solar sellsor both together can be used tocharge batteries or in direct heatingpurposes. Concerning stand-alonewindmill applications the ratedpower of which is below 10 kW,the use of permanent magnet

machines as a generator has beenstudied intensively during the lastdecades. Recently, LappeenrantaUniversity of Technology designand manufactured low-speedaxial flux permanent magnetgenerator designed to use in a1.6 kW windmill application.During the design of the generator,FLUX 3D finite element softwarewas an essential tool to verifythe performance of the designbefore building up the prototypegenerator. The prototype generatoris installed to the pilot power plant,which has been on operation sinceNovember 2003. Figure 1 presentsthe experimental power plant andillustrates the used generatorconcept.

 Axial flux PMM with duple layer 

concentrated winding

The generator was realized with aduple layer concentrated winding.Concentrated stator windings arean effective solution to reduceJoule’s losses in low-speedpermanent magnet machines, thusto improve the overall efficiency of the machine. By combining theconcentrated winding and axialflux permanent magnet machine,

which offers a high torque tovolume ratio, a high performanceelectrical machine is obtained.Short end-windings decrease alsothe overall external diameter of the axial flux machine. Therebythe overall space, required by thegenerator, is decreased which is ahighly desired feature for the windpower generators.

Permanent magnets are Nd-Fe-Bmagnets and are installed directlyon the surface of the solid iron rotor

disk. As a drawback of the singlesided construction, illustrated infigure 1, there appears quite alarge uncompensated attractiveforce between the rotor andthe stator, which has to takeinto account while design themechanics. According to theperformed 3D finite elementanalysis the attractive forcebetween the stator and rotor is6800 N for the generator underno-load condition with nominal airgap.

FE-model of themachine

For the point of the modeling, thegeometry of the axial flux machineis an actual 3D problem, whichcannot be reduced to the 2D planeif an accurate electromagneticanalysis is required. Thereby, themachine is modeled as a 3D problemby using FLUX 3D FE-software.The used FE-model is illustrated infigure 2 as well as the air gap side

view of actual prototype machine.Due to the used consternated 3-phase winding, one cannot modelonly one pole since one rotor poledoes not represent the symmetryon a stator side as it does with

conventional integral slot 3-phasewindings. In this case the wholegeometry was described eventhough the size of the FE-problemcomes very large. The totalamount of second order volumeelements used in a FE-modelwas 116878. Computations wereperformed with mechanical setsin order to model the rotation

and with circuit coupling. 3D FEAwas performed both under loadand no-load conditions giving

Figure 2. The used 3D FE-model and anactual generator stator presented from theair gap side. The used generator structureincludes 12 slots in a stator and 14 poles

in a rotor.

(continued on page 5)

Figure 1. Experimental wind power plantand the used single sided axial flux 

permanent magnet generator.

 - 4 -

N° 47 - January 2005 - CEDRAT - CEDRAT TECHNOLOGIES - MAGSOFT Corp.

SOFTWARE>>

7/27/2019 Axial Flux PM Generator

http://slidepdf.com/reader/full/axial-flux-pm-generator 2/2

Axial Flux Permanent Magnet Generator for 

Wind Power Applications. (continued) 

 Asko PARVIAI NEN, Lappeenranta University of Technology, Finland, Pentti KONTKANEN, Kylmätec Ky, Outokumpu, Finland .

information, for example, fromphase voltages, torque ripple andtorque production capability of thegenerator.

Results

Since the 3D-FE problemwas coupled to the circuit, itwas possible to analyse theperformance of the machinein actual operation conditionsby introducing a time transientFE-model. Figure 3 comparesthe measured no-load phasevoltage (for the hot machine)to the calculated one as well asmeasured and calculated phasecurrents under load condition. Thesimilarity between the obtainedcurvatures is excellent, howeverthe amplitudes of the measuredones are lower. This is mainlyrelated to the permanent magnetoperation temperature; it isslightly higher than the calculatedone, giving actually lower B

rfor

the Nd-Fe-B magnet than the usedone was in a FE-model.

One important property forthe direct-driven wind powergenerator is a torque quality.Low cogging torque is required inorder to allow the turbine starteasily even with low wind speeds.Secondly, low amplitude for thetorque pulsations is required alsounder load condition. Even thoughthe presented design uses totallyopen slots, which were introduceddue to the extreme simplicity of the manufacturing of the winding,

a low torque ripple was achieved.Figure 4 presents the obtainedelectromagnetic torque from thecomputation under load condition.The peak-to-peak value of torqueripple is around 3 % from the ratedtorque, which can be consideredas a good result for the structureused.

Conclusions

A direct drive axial flux PM generatordesigned to small-scale wind

power application was describedshortly. The 3D-finite elementanalysis, performed by using FLUX3D, was an essential tool to verifythe performance of the generatorbefore manufacturing of the

Figure 4. Electromagnetic torque of the generator with resistive load.

Figure 3. Measured andcalculated phase voltage

and measured andcalculated phase currents

with resistive load.

prototype machine. A comparisonbetween the measurements andthe results offered by the 3D-FEAshows good agreement. Eventhough the 3D-FE model for thefractionally wound PM machine isrelatively large, it can be solvedin a reasonable time and thusit offers very detail and usefulinformation for the designer aboutthe performance of the design.

 - 5 -

N° 47 - January 2005 - CEDRAT - CEDRAT TECHNOLOGIES - MAGSOFT Corp.

SOFTWARE>>